there is no better, they mean completely different things
about when – vs – about how much
you also didn’t provide any context for this
いつぐらい would only be when talking about time, but どのぐらい can be about time, amount of something, etc. imagine it’s like “how when” (not a real phrase in English but bear with me) vs “how much.”
In what context? First one I’d say is more used in relation to time or age. E.g. いつぐらいから待ってました？ or いつぐらいから一人でトイレに行けた？
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there is no better, they mean completely different things
about when – vs – about how much
you also didn’t provide any context for this
いつぐらい would only be when talking about time, but どのぐらい can be about time, amount of something, etc. imagine it’s like “how when” (not a real phrase in English but bear with me) vs “how much.”
In what context? First one I’d say is more used in relation to time or age. E.g. いつぐらいから待ってました？ or いつぐらいから一人でトイレに行けた？